The present invention relates to a radiation detector and, more particularly, to such a detector comprising an entrance screen which converts radiation into electrons and, accommodated in an envelope, an electron-optical system and an electron-detecting exit screen.
Detectors of this kind are known, for example, in the form of brightness intensifier tubes. The aim of brightness intensifier tubes, and more specifically image intensifier tubes, is to intensify information by converting a low-intensity entrance event into an optically faithful, high-intensity exit event. The detector has a converting function when instead of, for example, a light image an image which is carried by electro-magnetic radiation outside the visible spectral range or by corpuscular radiation is to be converted into an image which can be visually observed. The intensification of intensifying tubes is based mainly on an acceleration with, for example, from 10 to 30 kV of photo-electrons which are released in the entrance screen by incident radiation. A further brightness intensification is usually obtained by image reduction between the entrance screen and the exit screen, for example, by a factor from 10 to 50 measured in surface area. The optically faithful imaging in such tubes imposes severe specific requirements as regards the electron optical imaging system and imposes restrictions as regards the geometry of the entrance screen.
For the detection of individual photo-electrons, use could be made of a photo-multiplier tube comprising an extremely efficient photo-cathode and at least a first dynode having a high secondary emission enabling a detection within a period of about 100 nsec. The small entrance window of such tubes, however, makes it impossible to achieve a high sensitivity for these detectors.